For a test process, e.g., a high-temperature aging test process for improving stability and reliability of a display product, it is necessary to provide, by signal generators, aging test signals (including a power source signal and an image display signal) to the display product in waveforms. However, it is very difficult to ensure that each signal generator is capable of outputting the signal normally. If the signal generator outputs an abnormal signal, display abnormalities (e.g., ghost image and abnormal grayscale) may occur.